A touch-screen is a display operable to detect a presence and location of a touch within the display area. The term generally refers to touch or contact to the display of the device by a finger, hand or other passive objects, such as a stylus or pen.
A touch screen is typically made out of two transparent layers or plates, one on top of the other. Typically, a resilient material is used to keep the two layers separated. When enough pressure is applied to the surface of a touch-screen (for example, with a stylus or finger), the top layer is brought into contact with the bottom layer, i.e. a wiper contact is established between both layers or plates.
As shown in FIG. 1, a resistive touch-screen may have a back layer such as glass 10 with a uniform resistive coating 12, such as for example indium tin oxide (ITO) and a cover layer 14, such as a polyester film, additionally having a hard coat 16, wherein the cover layer 14 also has a resistive coating 18, with the resistive coatings 12, 18 separated by insulating microdot spacers 20. When the screen is touched, for example with a stylus 24, the force of the touch pushes the conductive coating 18 on the coversheet 14 against the coating 12 on the glass 10, allowing for an electrical wiper contact 22. The voltages produced may be analog representations of the position touched, which may be converted into digital X and Y coordinates. The shown screen also has a bezel 26.
Resistive touch-screens may for example be 4- or 8-wire touch-screens or 5- or 7-wire touch screens. Variations using more wires are possible.
4-wire and 8-wire touch screens are made out of two layers of a transparent resistive material that have a uniform surface resistance. For example, as shown in FIG. 2, a 4-wire resistive touch-screen may have a resistive x-plate 30 having a first and a second terminal for example in horizontal direction, which may be referred to as the first 32 and second x-terminals 34, and a resistive y-plate 36 having a first and a second terminal in vertical direction, which may be referred to as the first 38 and second y-terminals 40. Each plate has a resistance which may be represented divided into a first 42 and a second resistance 44 of the x-plate 30 and a first 46 and second resistance 48 of the y-plate 36 when locally bringing both plates into contact. This wiper contact has a contact resistance 50. A controller first applies a supply voltage to the x-plate 30. Upon touch, it probes the voltage received at the y-plate 36, which represents a left-right position or x-position. It then applies supply voltage to the y-plate 36 and probes from the x-plate 30 to calculate a y-position.
5-wire and 7-wire touch screens are made with one resistive layer and one conductive layer which only serves as a probe. For example, as shown in FIG. 3, a 5-wire resistive touch-screen may have an x-plate 52 with a first 54, a second 56, a third 58, and a fourth x-terminal 60. The y-plate or cover-layer (not shown) may only serve as a probe and may have a y-terminal. Touching the y-layer may result in a wiper contact 62 with the x-plate 52. A controller first applies a supply voltage to the first and third x-terminals 54, 58 and grounds the second and the fourth x-terminals 56, 60, causing voltage to flow uniformly across the screen from the top to the bottom. Upon touch, it reads the Y voltage from the coversheet or y-plate at the y-terminal. Then the controller applies the supply voltage to the first and the second x-terminals 54, 56 and grounds the third and fourth x-terminals 58, 60, and reads the X voltage from the y-terminal again.